Photochemistry comprises chemical reactions of atoms or molecules that have been electronically excited by absorption of light with wavelength typically in the range of 200 nm to about 700 nm. There are several advantages of this kind of chemistry which can be present individually or in various combinations, depending on the particular reaction(s) at issue: (1) Light may be considered as a highly specific and ecologically clean “reagent”; (2) Photochemical methods can offer less “aggressive” routes in chemical synthesis than thermal ones; (3) Photochemical reactions may contribute to the safety of industrial processes, because they are generally performed at, or below, room temperature; (4) photo-chemically reacting organic compounds do not require any protecting groups; and (5) Many conventional syntheses can be shortened by inserting photochemical reaction steps.
Despite those known advantages, photochemical processes are rare on industrial scale. There are several reasons for the limited use in industry.
One is difficult scale-up. Today the best scale-up is generally to enlarge the lab reactor. Pilot plant experiments are done with a production size lamp and the scale up is then done by adding more lamps to the reactor or adding more reactors, which is very expensive. The reactors vary in the way the lamps are placed, in the relation of lamp power to lamp distance from the reactant(s), and in the number of lamps.
Another limitation is due to economic considerations. Photo reactions consume significant amounts of energy. This energy consumption can be divided into two categories: One part is related to chemistry itself and defined by the quantum yield, which is a constant. The other part is caused by the photoreactor setup. In a typical photoreactor and photoreaction, on average, only 10% of the lamp energy is used in the reaction, while 90% produces heat, which must be extracted from the system by cooling. This is normally done with a cooling jacket in which water is circulating. This increases the energy cost.
Another drawback is the maintenance which has to be performed very regularly, since the life of the lamps is limited (typically to 2000-6000 hours). The exchange of the lamps is typically extremely time consuming, requiring significant manual labor.
A photo reactor offering improvements in scale up or energy efficiency, or maintenance requirements or any combination of these is thus desirable.